(1) Field of the Invention
The present invention relates to positive dobby machines constituted to drive heald frames positively in both upward and downward directions, and more specifically to a positive dobby machine where mechanism in a vertical lever, upper and lower hooks pivotally connected respectively to upper and lower ends of the vertical lever, and upper and lower return knives acting to push back both ends of the vertical lever.
(2) Description of the Prior Art
In positive dobby machine in prior art, a command device constituted to generate command corresponding to fabric pattern acts to rotate upper and lower hooks pivotally connected respectively to upper and lower ends of a vertical lever. Therefore upper and lower return knives opposed respectively to upper and lower ends of the vertical lever are contacted with both ends of the vertical lever directly so as to transmit movement. If upper and lower return knives were constituted to push the vertical lever through upper and lower hooks, the pushing force would produce revolution resistance in the upper and lower hooks and rotation of both hooks according to the command device would be difficult.
Since the revolution center of the vertical lever and upper and lower return knives cannot coincide with each other, surface contact state of the vertical lever with the return knife cannot be held throughout whole stroke of transmission. Therefore, partial cylindrical surfaces are usually formed on upper and lower ends of the vertical lever, and pressure receiving surfaces of upper and lower knives opposed to respective cylindrical surfaces are formed in flat planes, so that the cylindrical surface and the flat plane are contacted with each other in line contact state. In such constitution, pressure per unit area between the vertical lever and the return knife becomes very large. Moreover sliding phenomenon between contact portions because of non-coincidance of the revolution center increases abrasion of the contact portions. High-speed operation of positive dobby machines therefore has been obstructed.